The Life Processes of the Individual 155 
attachment of the bee’s wings to the thorax has been in¬ 
vestigated by Snodgrass ( lx . pp. 61-63). 
The motion of the wings in flight is in four directions, up, 
down, forward and backward, and the combination of these 
movements causes the wing tips to describe the course of a 
figure 8, if the insect is held stationary. In flight, the 8 
is of course modified. The hind wings are small and are 
attached by hooks on their anterior margins to thickenings 
on the margin of the front wings. They are not provided 
with large flight muscles of their own but are carried along 
by the action of the powerful muscles in the mesothorax 
which propel the fore 
wings. 
The muscles of flight 
are in four sets, cor¬ 
responding to the four 
directions of wing move¬ 
ment. The chief muscles 
are not attached directly 
to the bases of the wings, 
as in dragonflies, but the 
wings are moved into 
the right position by muscles situated inside the pleura of 
the two thoracic segments. After the wings are in position 
for flight, the compression of the thorax by the vertical 
muscles lowers the dorsum and raises the wing while the 
contraction of the longitudinal muscles raises the dorsum 
and lowers the wing. The vertical muscles are therefore 
the elevators and the longitudinal muscles the 'depressors. 
The movements of the wings during flight is therefore 
produced mainly by changes in the shape of the thorax. 
The forward and backward movements are accomplished 
by the action of the muscles on the pleurum, acting directly 
on the bases of the wings. 
Because of the enormous development of the two main 
sets of flight muscles, bees are capable of strong and rapid 
flight. They are also capable of arresting progress suddenly 
Fig. 80. — Fore and hind wings. 
